text: Some of the most advanced molecular switches are based on mechanically-interlocked molecular architectures where the bistable states differ in the position of the macrocycle. In 1991 Stoddart devices a molecular shuttle based on a rotaxane on which a molecular bead is able to shuttle between two docking stations situated on a molecular thread. Stoddart predicts that when the stations are dissimilar with each of the stations addressed by a different external stimulus the shuttle becomes a molecular machine. In 1993 Stoddart is scooped by supramolecular chemistry pioneer Fritz Vögtle who actually delivers a switchable molecule based  not on a rotaxane but on a related catenane
  This compound is based on two ring systems: one ring holds the photoswichable azobenzene ring and two paraquat docking stations and the other ring is a polyether with to arene rings with binding affinity for the paraquat units. In this system NMR spectroscopy shows that in the azo trans-form the polyether ring is free to rotate around its partner ring but then when a light trigger activates the cis azo form this rotation mode is stopped
  Kaifer and Stoddart in 1994 modify their molecular shuttle such a way that an electron-poor tetracationic cyclophane bead now has a choice between two docking stations: one biphenol and one benzidine unit. In solution at room temperature NMR spectroscopy reveals that the bead shuttles at a rate comparable to the NMR timescale, reducing the temperature to 229K resolves the signals with 84% of the population favoring the benzidine station. However, on addition of trifluoroacetic acid, the benzidine nitrogen atoms are protonated and the bead is fixed permanently on the biphenol station. The same effect is obtained by electrochemical oxidation (forming the benzidine radical ion) and significantly both processes are reversible.
  In 2007 molecular shuttles were utilized in an experimental DRAM circuit. The device consists of 400 bottom silicon nanowire electrodes (16 nanometer (nm) wide at 33 nm intervals) crossed by another 400 titanium top-nanowires with similar dimensions sandwiching a monolayer of a bistable rotaxane depicted below:
  Each bit in the device consists of a silicon and a titanium crossbar with around 100 rotaxane molecules filling in the space between them at perpendicular angles. The hydrophilic diethylene glycol stopper on the left (gray) is specifically designed to anchor to the silicon wire (made hydrophilic by phosphorus doping) while the hydrophobic tetraarylmethane stopper on the right does the same to the likewise hydrophobic titanium wire.  In the ground state of the switch, the paraquat ring is located around a tetrathiafulvalene unit (in red) but it moves to the dioxynaphthyl unit (in green) when the fulvalene unit is oxidized by application of a current. When the fulvalene is reduced back a metastable high conductance 1 state is formed which relaxes back to the ground state with a chemical half-life of around one hour. The problem of defects is circumvented by adopting a defect-tolerant architecture also found in the Teramac project. In this way a circuit is obtained consisting of 160,000 bits on an area the size of a white blood cell translating into 10 bits per square centimeter.